Heretofore, various types of mooring sytems have been utilized for mooring vessels and buoys floating on the surface of the sea. Weights or other anchors on the seabed or sea floor have commonly been used to secure the lower end of various lines or cables running to a buoy or vessel.
Additionally, so-called semi-submersible drilling vessels or drill ships have been provided heretofore for drilling in deep seas or oceans. Such drill ships have been moored over a well and are normally anchored to the seabed. In determining the design of a mooring system, wind, wave and current forces must be analyzed. The calculation of wind and current forces on a vessel are normally easily calculated. However, wave forces are more dificult to evaluate as waves tend to load the mooring lines and fluctuations in tension as may result from storms may be substantial.
A mooring system acts as a spring to resist offsetting of a vessel or other structure being moored or anchored. As in a spring, the restoring force increases with an increasing offset. The rate at which this force increases is conventionally referred to as the hardness or stiffness of a mooring sytem. A restoring force calculation is normally made by using the catenary equation which describes a line that is suspended at its two ends and allowed to sag under its own weight. The hardness of a mooring system decreases with water depth. Mooring lines have normally included chains, cables, and wire ropes of various designs and sizes.
Submerged acoustic arrays for subsea listening purposes have existed for many years. Such arrays must periodically be brought to the sea surface for maintenance. Before the invention described below, it has been a costly time consuming procedure to bring such arrays to the surface and to redeploy them safely at a submerged position.
Buoyant modules have been used in subsea operations for attachment to risers in the offshore drilling art between the sea floor and the surface to decrease the tension required at the surface. These modules have included thin-walled air cans or fabricated syntactic foam modules that are strapped to the riser. Air cans have a predictable buoyancy and the buoyancy can be controlled from the surface by displacing water from the cans by air pressure. Syntactic foam modules have been used with various compositions of foam.